Construction workers prepare a section of curved retaining wall for installation inside the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

Construction workers prepare a section of curved retaining wall for installation inside the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large ... more

James Martin runs the control center of Big Alma, as the boring machine drills below Union Square in the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

James Martin runs the control center of Big Alma, as the boring machine drills below Union Square in the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. ... more

Construction workers prepare a section of curved retaining wall for installation inside the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

Construction workers prepare a section of curved retaining wall for installation inside the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large ... more

Construction workers prepare a section of curved retaining wall for installation inside the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

Construction workers prepare a section of curved retaining wall for installation inside the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large ... more

Assistant construction superintendent Andy Granger looks south from a tunnel for Muni's Central Subway project below Union Square in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

Assistant construction superintendent Andy Granger looks south from a tunnel for Muni's Central Subway project below Union Square in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, ... more

Muni project manager John Funghi (left) and spokesman Paul Rose ride aboard a transport vehicle inside the northbound tunnel of the transit agency's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

Drilling work continues in the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

Drilling work continues in the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way ... more

R.W. Carpenter works on the curved retaining wall for the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

R.W. Carpenter works on the curved retaining wall for the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom ... more

A transport vehicle emerges from the northbound tunnel of Muni's Central Subway construction project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

A transport vehicle emerges from the northbound tunnel of Muni's Central Subway construction project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are ... more

Visitors ride aboard a transport vehicle in the northbound tunnel of Muni's Central Subway tunneling project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

Visitors ride aboard a transport vehicle in the northbound tunnel of Muni's Central Subway tunneling project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom ... more

Muni's project manager John Funghi inspects curved sections of retaining wall that will line tunnels of the Central Subway in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

Muni's project manager John Funghi inspects curved sections of retaining wall that will line tunnels of the Central Subway in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big ... more

Visitors are updated on construction inside the northbound tunnel of Muni's Central Subway in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

Visitors are updated on construction inside the northbound tunnel of Muni's Central Subway in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding ... more

R.W. Carpenter works on the curved retaining wall for the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

R.W. Carpenter works on the curved retaining wall for the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom ... more

A construction worker - and Seattle Seahawks fan - displays a "12th Fan" banner on Big Alma in the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

A construction worker - and Seattle Seahawks fan - displays a "12th Fan" banner on Big Alma in the northbound tunnel of Muni's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two ... more

Muni project manager John Funghi views construction work underway in the northbound tunnel of the transit agency's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring machines, Big Alma and Mom Chung, are grinding their way north from Fourth and Bryant streets towards Chinatown and North Beach. less

Muni project manager John Funghi views construction work underway in the northbound tunnel of the transit agency's Central Subway project in San Francisco, Calif. on Saturday, March 8, 2014. Two large boring ... more

Traveling from South of Market, past Moscone Convention Center and to the far end of Union Square on Saturday morning was an other-worldly experience: no cars, no stores, no tall buildings, no crowded sidewalks, no panhandlers.

Everything was gray. And noisy - a loud constant buzz bordering on a roar mixed with the incessant metallic hammering sound interspersed with occasional beeps. Even the weather was odd. It was warm and humid, yet dry. Still, the pavement underfoot was covered in a sticky layer of mud.

This trip, seemingly out of a science fiction novel, was a tour of the under-construction northbound bore of the Central Subway - the Municipal Transportation Agency's $1.6 billion transit link between the Caltrain station and Chinatown.

The line will stretch 1.7 miles, with a twin-bore tunnel going underground where Interstate 80 crosses Fourth Street. An above-ground station will be built at Fourth and Brannan streets with subterranean stations near Moscone Center at Fourth and Folsom streets, at Union Square and in Chinatown at Stockton and Washington streets, where the tracks will end.

But the tunnel will extend to Powell Street and Columbus Avenue, the site of the old Pagoda Palace Theater in North Beach, where the two tunnel-boring machines will be plucked from the ground and an extension might someday be built.

For now, the two machines - each longer than a football field and weighing 750 tons - are steadily and surreptitiously gnawing 20-foot wide tunnels beneath one of the most-congested parts of the city. Tunneling crews work five days a week, 12 hours a day, with maintenance work taking place when they're not digging.

Borers named after women

Mom Chung, the machine named for the nation's first American-born female Chinese physician, got a head start in July and is now at Stockton and Clay streets in Chinatown. Big Alma, dedicated to socialite and philanthropist Alma de Bretteville Spreckels, started work in November and has made it past Union Square. She's now sitting beneath the Nike store.

Mike Sinon, safety manager for contractor Barnard Impreglio Healy, said it's a tradition as well as a safety precaution to name the tunneling machines - usually after women.

"It's bad juju to not name tunnel-boring machines," he said. "It's done all over the world."

Imperceptible on surface

So far, it seems to be working. John Funghi, the MTA's Central Subway project manager, said the work has been imperceptible on the surface, even when passing beneath some of the city's busiest areas, such as Fourth and Market streets, where the boring machines had to dig beneath Old Navy and Forever 21.

"You could be in the Nike store shopping and you wouldn't feel a thing," Funghi said. "We've been very fortunate. Nothing on the surface has moved. We crossed under BART without stopping or even disrupting service."

The boring machines are made of three parts: a rotating cutting wheel, known as the cutter head, attached to a steel cylinder that serves as a shield and operations center and 300 feet of tunneling equipment. The machine spews a conditioning foam that softens the rock and dirt, which is ground up and inhaled by the cutter head and transported by a giant screw onto conveyor belts that haul the diggings out of the tunnels.

As they lurch forward, 5 feet at a time, the machines install prefabricated curved concrete slabs that form the tunnel's linings. Workers bolt them together and inject caulk behind them, and the boring machines press forward.

The machines excavate and build about 50 feet of tunnel a day, Funghi said. Big Alma is quicker at 54 feet a day compared to Mom Chung's 44-foot average, but Mom Chung holds the performance record of 96 feet in a single day.

As the machines continue to chew their way north, construction crews are working on the North Beach shaft where the machines will be removed. Mom Chung is expected to be pulled out in May with Big Alma following a few months later.

The tunneling may be done this summer, but it will be another 4 1/2 years before passengers can ride the Central Subway in 2019. Construction crews need to build the subway stations, lay the rail beds and install the tracks, overhead power systems and train control and communication equipment.

Metro system connector

"This will be the Metro system connecting north and south, which has not been connected by rail so far," said Paul Rose, an MTA spokesman. "It's connecting Chinatown, one of the densest areas not only in the city but in the state and country."

He said the project is on schedule and on budget.

While the tunneling may be going smoothly, subway opponents, particularly in North Beach, continue to fight the project, which they contend is disrupting businesses. Howard Wong, a member of Save Muni, which opposes the subway, said two North Beach restaurants have closed, in part because of the noise and dust from construction, and several Chinatown businesses have seen huge drops in business due to the closure of Washington Street at Stockton Street.

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